Thirty is the new 40. Just ask the detergent supremos and giant clothing retailers pushing the environmental credentials of the great low-temperature wash.

It began last year with Ariel (the market leader) and you can see what inspired them. We wash hotter in the UK than they do on the Continent, thus wasting 1.6 billion kilowatt hours of energy a year (but how much of this is the legacy of the industry emphasis on whiter than white whites?)

It almost adds up. Studies on jeans, a polyester blouse and a T-shirt show that laundering accounts for 60-80 per cent of the garment's total environmental impact. But there's no denying it's extremely clever marketing, too, allowing detergent manufacturers to up their planet-saving credentials without particularly changing a product's ingredients (phosphates and optical brighteners are especially unfriendly to watercourses).

In theory, lower the temperature of your wash and you shrink your carbon footprint (though helpfully not the garment). Which would seem to be extremely helpful, because we are washing like fury; the average load used to be 4.5kg, but now it's nearer 8kg.

I say 'in theory' because if you focus on toxicity in production, even clean cotton looks grubby (cotton soaks up 25 per cent of all the world's agrichemicals). Then washing at 30 looks a less satisfactory answer.

Not that this is stopping Asda from embracing the low-wash mantra. From March all George clothes will wear 'wash at 30' labels. Cue planet-saving statistics: if all the 237 million garments Asda sells every year were washed at 30, enough power to run 5,300 TVs would be saved.

Great. But this narrow focus doesn't really look at the amount of energy and resources that would be saved if we stopped stuffing our wardrobes with too many clothes. Because actually, what lurks in our obese wardrobes are 2.4 billion unwanted items, each of which took huge amounts of resources to be made, shipped and brought to point of sale.

Meanwhile Dr Kate Fletcher quotes Dutch research in her book Sustainable Fashion and Textiles which shows that the average garment stays on the body for 44 days and is worn for between 2.4 and 3.1 days between washing. As we buy more these times are decreased and wash days increase. So a sustainable laundry manoeuvre might also include learning to love stains; a little dried egg or butter grease never killed anybody.

So yes, lower your wash temperature, but also buy less and extend wear time in between washes. Without the last two stages, you're just setting the dial to green spin.